The present invention relates to an organic photoconductor. More particularly, this invention relates to an organic photoconductor which employs an inexpensive cylindrical electrically conductive support and is excellent in electrical properties and image quality despite the use of the inexpensive support.
In general, an organic photoconductor is composed of an electrically conductive support and formed thereon a photosensitive layer comprising a photoconductive material. Generally employed as this electrically conductive support is a cylindrical electrically conductive support made of a metal, e.g., aluminum. Organic photoconductors in frequent use have a function allocation type multilayered photosensitive layer comprising a charge-generating layer and a charge-transporting layer.
To cope with enlargement in sensible-wavelength region or with special development techniques such as multiple exposure, organic photoconductors having a multilayered charge-generating layer consisting of two or more layers have been proposed in, e.g., JP-A-50-75042 (U.S. Pat. No. 3,992,205), JP-A-63-38942, JP-A-63-142356, JP-A-63-292136, JP-A-1-185635, and JP-A-6-27689. (The term xe2x80x9cJP-Axe2x80x9d as used herein means an xe2x80x9cunexamined published Japanese patent application.xe2x80x9d)
In most of such organic photoconductors, the external surface of the cylindrical electrically conductive support has conventionally been finished by turning by means of a precision lathe or the like. It is usually necessary that the external surface of such cylindrical electrically conductive supports be regulated so as to have a surface roughness not higher than a specific value in order to avoid image defects. It should be noted in this connection that suitability of a cylindrical electrically conductive support for use in an organic photoconductor can be more infallibly judged by use of the index of surface area as defined by the following equation (I) as an index to surface roughness than by the center-line average roughness Ra or the maximum surface roughness Rmax which are both frequently used as an ordinary index to surface roughness.
(Sa/Sm)xe2x88x921xe2x80x83xe2x80x83(I)
(In equation (I), Sa is the actual surface area of the external surface of the cylindrical electrically conductive support and Sm is the theoretical surface area thereof calculated on the assumption that the support is an ideal cylinder.)
This index of surface area can be easily measured, for example, with an apparatus such as scanning electron stereomicroscope ERA-8000, sold by ELIONIX K.K.
The commonly used cylindrical electrically conductive supports which have undergone surface finishing by means of turning have a value of the index of surface area smaller than 0.01, in most cases 0.005 or smaller. It has been found that in an organic photoconductor comprising a cylindrical electrically conductive support having a roughly finished surface with a value of the index of surface area of 0.01 or smaller and formed thereon a conventional photosensitive layer consisting of a charge-generating layer and a charge-transporting layer, the possibility of print defects appearing on images is fairly high.
In recent years, cylindrical electrically conductive supports which have undergone surface finishing by means of not turning but precision drawing, ironing, impacting, or the like for the purpose of production-cost reduction have also become available. However, such supports have surface properties utterly different from those of conventional supports; specifically, they inevitably have both groove defects in the form of streaks parallel to the cylinder axis and depression defects in the form of holes. Because of such surface properties, the cylindrical electrically conductive supports produced through any of the non-turning surface-finishing techniques have a value of the index of surface area of 0.01 or larger, in most cases 0.02 or larger. Organic photoconductors comprising such a cylindrical electrically conductive support and formed thereon a conventional photosensitive layer consisting of a charge-generating layer and a charge-transporting layer are apt to cause image defects as different from the organic photoconductors employing a cylindrical electrically conductive support finished by turning, which tend less to cause such defects. This drawback has been an obstacle to the spread of the cylindrical electrically conductive supports finished by non-turning processing.
On the other hand, it is known that in function allocation type multilayered organic photoconductors, the properties thereof vary considerably depending on electrical junction between the cylindrical electrically conductive support and the charge-generating layer. For example, in an ordinary non-rectifying (so-called ohmic type) junction, in which electric current is proportional to electric field, charges are always injected from the cylindrical electrically conductive support during electrification, resulting in a diminished surface potential to significantly reduce the organic-photoconductor""s ability to be charged. In this case, the ability to be charged can be enhanced by forming an electrical barrier layer between the photosensitive layer and the cylindrical electrically conductive support to inhibit the injection of charges from the cylindrical electrically conductive support. In contrast, organic photoconductors having a junction of the so-called Schottky type, for example, organic photoconductors employing a combination of a cylindrical electrically conductive support made of a metal having a small work function, such as aluminum, and a phthalocyanine compound as a charge-generating material, are known to be inhibited from undergoing charge injection from the support due to the rectifying function of the junction and to show relatively high ability to be charged even without a barrier layer.
However, in this case also, local drops in potential occur due to the defects and crystallized impurities scatteringly present on the surface of the cylindrical electrically conductive support and due to impurities in the coating, etc. It is known that these local potential drops may result in print defects on images, especially in reversal development. To cope with this problem, a barrier layer is generally formed.
Prior art organic photoconductors designed to have improved functions by means of a barrier layer have the following drawbacks. For example, in the case of organic photoconductors employing a barrier layer made of an electrical insulating polymer, materials usable as the barrier layer are considerably limited, because they not only are required to have moderate barrier properties and adherence but also should satisfy requirements including freedom from dissolution during coating for forming an overlying layer. Examples of conventionally generally used polymeric materials which satisfy such requirements include hydrophilic resins such as casein, poly(vinyl alcohol), polyamides, and poly(vinyl butyral). Such barrier layers have a problem that too small a film thickness results in insufficient barrier properties and insufficient hiding of substrate defects, while too large a film thickness undesirably results in inhibition of charge injection from the photosensitive layer to the support, leading to a decrease in sensitivity and an increase in residual potential. Another problem is that since those barrier layers are made of a highly hydrophilic material which is apt to absorb water, they upon water absorption not only undergo a decrease in barrier property to impair photoconductor properties but also frequently cause other troubles such as insufficient adhesion to the photosensitive layer.
A technique of using as a barrier layer a film of an insulating inorganic compound such as, e.g., Al2O3 or SiO2 is disclosed in, e.g., JP-A-2-7070 and JP-A-3-192265. This technique however has drawbacks in that since such an insulating inorganic-compound film should be formed by a chemical treatment or a technique such as vacuum deposition or sputtering, not only much time is required for barrier layer formation but the formation of such a barrier layer results in a considerably increased production cost, etc. In addition, like barrier layers of the resinous coating type, the inorganic barrier layers have unsolved problems of decrease in sensitivity and increase in residual potential.
Furthermore, a technique of using as a barrier layer a layer comprising white inorganic particles of TiO2, ZnO, or the like dispersed in a resin is disclosed in, e.g., JP-A-57-81269 and JP-A-59-139967. However, this prior art barrier layer not only has basically the same drawbacks as the resinous coating type barrier layers, but also necessitates extremely troublesome regulations concerning the purity, particle diameter, and surface state of the inorganic material, blending ratio thereof to the resin, etc. so as to obtain organic photoconductor properties suitable for practical use.
As described above, the conventionally employed general method for improving functions of multilayered organic photoconductors, in particular charge acceptance and image properties, has been used to form an independent barrier layer. However, the prior art photoconductors having a barrier layer have insufficient properties for application to cylindrical electrically conductive supports which have not undergone surface finishing by turning and have a large value of the index of surface area.
An object of the present invention is to provide an organic photoconductor which employs an inexpensively produced cylindrical electrically conductive support having a surface finished by a technique other than turning and having a value of the index of surface area of from 0.01 to 0.10 and does not necessitate a barrier layer, and which is comparable or superior in charge acceptance and image properties to organic photoconductors employing a conventional cylindrical electrically conductive support finished by turning and having a value of the index of surface area below 0.01.
As a result of investigations made by the present inventor on a large number of materials and layer constitutions in order to satisfy various properties required of an organic photoconductor, the present invention has been achieved.
To accomplish the above object, the present invention provides an organic photoconductor comprising a cylindrical electrically conductive support and, formed thereon in this order, a charge-generating layer and a charge-transporting layer, the external surface of said cylindrical electrically conductive support having an index of surface area as defined by the following equation (I) of from 0.01 to 0.10, and said charge-generating layer having a multilayer structure comprising a charge-generating layer having n-type semiconductor characteristics and a charge-generating layer having p-type semiconductor characteristics:
(Sa/Sm)xe2x88x921xe2x80x83xe2x80x83(I)
wherein Sa is the actual surface area of the external surface of said cylindrical electrically conductive support and Sm is the theoretical surface area thereof calculated on the assumption that said support is an ideal cylinder.